1. Field of the Invention
The present invention relates to non-aqueous pigment dispersions of Naphthol AS pigments, especially to inkjet inks of Naphthol AS pigments exhibiting high dispersion quality and stability.
2. Description of the Related Art
Pigment dispersions are made using a dispersant. A dispersant is a substance for promoting the formation and stabilization of a dispersion of pigment particles in a dispersion medium. Dispersants are generally surface-active materials having an anionic, cationic or non-ionic structure. The presence of a dispersant substantially reduces the required dispersing energy. Dispersed pigment particles may have a tendency to re-agglomerate after the dispersing operation, due to mutual attraction forces. The use of dispersants also counteracts this re-agglomeration tendency of the pigment particles.
The dispersant has to meet particularly high requirements when used for inkjet inks. Inadequate dispersing manifests itself as increased viscosity in liquid systems, loss of brilliance and/or hue shifts. Moreover, particularly good dispersion of the pigment particles is required to ensure unimpeded passage of the pigment particles through the nozzles of the print head in an inkjet printer, which are usually only a few micrometers in diameter. In addition, pigment particle agglomeration and the associated blockage of the printer nozzles has to be avoided during the standby periods of the printer.
Many polymeric dispersants contain in one part of the molecule so-called anchor groups, which adsorb onto the pigments to be dispersed. In a spatially separate part of the molecule, polymeric dispersants have polymer chains sticking out whereby pigment particles are made compatible with the dispersion medium, i.e., stabilized.
In aqueous inkjet inks, the polymeric dispersants generally contain hydrophobic anchor groups exhibiting a high affinity for the pigment surface and hydrophilic polymer chains for stabilizing the pigments in the aqueous dispersion medium.
The preparation of good thermally stable dispersions with submicron particles is more difficult for non-aqueous inkjet inks, such as solvent based, oil based and radiation curable inkjet inks. The pigments are especially difficult to disperse when they have a non-polar surface.
These problems have led to the design of very specific polymeric dispersants wherein the anchor groups are pigment derivatives. For example, EP 0763378 A (TOYO INK) discloses a pigment composition including a non-aqueous type pigment dispersing agent having a portion which has a high affinity with a pigment and which has at least one type selected from the group consisting of an organic dye, anthraquinone and acridone only at a terminal end or at both terminal ends of at least one polymer selected from a linear urethane polymer and a linear acrylic polymer, and a pigment.
Another approach for dispersing pigments with non-polar surfaces in non-aqueous dispersion media is changing the surface to a more polar surface by addition of compounds known as dispersion synergists. A dispersion synergist is a compound that promotes the adsorption of the polymeric dispersant on the surface of the pigment. It is suggested that the synergist should possess the pigment structure substituted by one or more sulphonic acid groups or ammonium salts thereof.
U.S. Pat. No. 4,461,647 (ICI) discloses a dispersion of a pigment in an organic liquid containing a water-insoluble asymmetric disazo compound including a central divalent group free from acidic and other ionic substituents linked through azo groups to two monovalent end groups wherein one end group, the first, is free from acidic and other ionic substituents and the other end group, the second, carries a single substituted ammonium-acid salt group.
U.S. Pat. No. 4,057,436 (ICI) discloses pigment dispersions in organic liquids using polymeric or resinous dispersing agents in the presence of a substituted ammonium salt of a colored acid wherein there are between 16 and 60 carbon atoms contained in at least 3 chains attached to the N-atom of the substituted ammonium ion.
U.S. Pat. No. 6,641,655 (AVECIA) discloses the use of a diquaternary ammonium salt of a colored acid as fluidizing agent wherein the diquaternary ammonium cation contains two or more nitrogen atoms.
Although these dispersion synergists work fine for some pigments, many other pigments cannot be dispersed to an acceptable quality in a non-aqueous medium. This is the case for Naphthol AS pigments, for which it is difficult to obtain stable non-aqueous pigment dispersions, especially stable non-aqueous inkjet inks.
JP 59-227951 (TOYO INK) discloses diazo compounds prepared by coupling a 3-phenylcarbamoyl-2-Naphthol compound with a diazonium salt of an arylamine wherein these diazo compounds include carboxylic acid groups and salts thereof. The diazo compounds are used as dispersants for a wide range of pigments in offset inks, gravure inks and paints.
For consistent image quality, inkjet inks require a dispersion stability capable of dealing with high temperatures (above 60° C.) during transport of the ink to a customer, jetting at elevated temperatures and changes in the dispersion medium of the inkjet ink during use, for example, evaporation of solvent and increasing concentrations of humectants, penetrants and other additives.
Therefore, it is highly desirable to be able to manufacture pigment inkjet inks using Naphthol AS pigments in a non-aqueous medium exhibiting a high dispersion quality and stability.